This is not a good idea. I see many bins full to brim every week in my neighbourhood. I would also comment that in the summer parts of Vancouver could smell like a garbage dump if pickup was every two weeks and not to mention the potential for an increase in the rat population. That a bin is only half full is immaterial as the garbage truck still has to go down the lane irrespective of the level of garbage in a bin.

Garbage needs to be collected every week. If you find you garbage cart is half empty all the time, GET A SMALLER CART! Also if you live a non social life with no visitors, no family or celebrations, maybe you have no need for a larger cart.
But you can plan ahead based on occasional demand.

What matters more, efficiency wise, would seem to be how full the trucks are. If the trucks are filling up then cutting the service to semimonthly would possibly increase wasted driving.

The City is looking into this with the new Food Scrap Program - they are looking for 2011 to have weekly Yard Trimming/Food Scrap collection and bi-weekly garbage collection.